Curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione]
is the major yellow pigment of turmeric, a commonly used spice, derived from the rhizome of the herb Curcuma longa Linn. In the Indian subcontinent and Southeast Asia, turmeric has traditionally been used as a treatment for inflammation, skin wounds, and tumors. Clinical activity of curcumin is yet to be confirmed; however, in preclinical animal models, curcumin has shown cancer chemo preventive, antineoplastic and anti-inflammatory properties1. Especially interesting is its ability to prevent the formation of carcinogen-induced intestinal premalignant lesions and malignancies in rat2,3 and in the multiple neoplasia (Min/+) mouse4, a genetic model of the human disease familial adenomatous polyposis. Curcumin acts as a scavenger of oxygen species such as hydroxyl radical, superoxide anion and singlet oxygen5,6,7 and interferes with lipid peroxidations8,9. Curcumin suppresses a number of key elements in cellular signal induction pathways pertinent to growth, differentiation and malignant transformations. Among signaling events inhibited by curcumin are protein kinases10, c-Jun/AP-1 activation11, prostaglandin biosynthesis12 and activity and expression of the enzyme cyclooxygenase-213,14. This latter property is probably mediated by the ability of curcumin to block activation of the transcription factor NF-κB at the level of the NF-κB inducing kinase/IKKα/β signalling complex15.
Curcumin directly inhibits cyclooxygenase-2 and also inhibits the transcription of the gene responsible for its production. Cyclooxygenases (COX) catalyze the synthesis of prostaglandins (PGs) from arachidonic acid. There are two isoforms of COX, designated COX-1 and COX-2. COX-1 is expressed constitutively in most tissues and appears to be responsible for housekeeping functions16 while COX-2 is not detectable in most normal tissues but is induced by oncogenes, growth factors, carcinogens and tumor promoters17,18,19. Several different mechanisms account for the link between COX-2 activity and carcinogenesis.
Curcumin is not simply an alternative to non-steroidal anti-inflammatory drugs (NSAIDS), which also have anti-inflammatory and cancer chemopreventive properties. This is so because COX is a bifunctional enzyme with cyclooxygenase and peroxidase activities. Aside from being important for PG synthesis, the peroxidase function contributes to the activation of procarcinogens. Therefore, the failure of NSAIDS to inhibit the peroxidase function of COX potentially limits their effectiveness as anticancer agents. Curcumin, in contrast, down-regulates levels of COX-2 and thereby decreases both the cyclooxygenase and peroxidase activities of the enzyme.
Curcumin is among the few agents to block both the COX and LOX (lipoxygenase) pathways of inflammation and carcinogenesis by directly modulating arachidonic acid metabolism. In a study to evaluate the effect of curcumin on the metabolism and action of arachidonic acid in mouse epidermis, it was found that topical application of curcumin inhibited arachidonic acid-induced ear inflammation in mice20. Curcumin (10 μM) inhibited the conversion of arachidonic acid to 5- and 8-hydroxyeicosatetraenoic acid by 60% and 51%, respectively (LOX pathway) and the metabolism to PGE2, PGF2α and PGD2 by 70%, 64% and 73%, respectively (COX pathway). In another study, dietary administration of 0.2% curcumin to rats inhibited azoxymethane-induced colon carcinogenesis and decreased colonic and tumor phospholipase A2, phospholipase Cγ1, and PGE2 levels21. In this study, dietary curcumin also decreased enzyme activity in the colonic mucosa and tumors for the formation of PGE2, PGF2α, PGD2, 6-keto-PGF2α and thromboxane B2 via the COX system and production of 5(S)-, 8(S)-, 12(S)-, and 15(S)-hydroxy-eicosatetraenoic acid via the LOX pathway was also inhibited.
Despite this impressive array of beneficial bioactivities, the bioavailability of curcumin in animals and man remains low. In rodents, curcumin demonstrates poor systemic bioavailability after p.o. dosing22 which may be related to its inadequate absorption and fast metabolism. Curcumin bioavailability may also be poor in humans as seen from the results of a recent pilot study of a standardized turmeric extract in colorectal cancer patients23. Indirect evidence suggests that curcumin is metabolized in the intestinal tract. Curcumin undergoes metabolic O-conjugation to curcumin glucuronide and curcumin sulfate and bioreduction to tetrahydrocurcumin, hexahydrocurcumin and hexahydrocurcuminol in rats and mice in vivo24,25 in suspensions of human and rat hepatocytes26 and in human and rat intestine27. Metabolic conjugation and reduction of curcumin was more in human than in rat intestinal tissue. It has been suggested that the intestinal tract plays an important role in the metabolic disposition of curcumin. This is based predominantly on experiments in which [3H] labeled curcumin was incubated with inverted rat gut sacs28. This was later confirmed in intestinal fractions from humans and rats. Intestinal mucosa, as well as liver and kidney tissue from the rat, can glucurodinate and sulfate curcumin, as judged by the analysis of differential amounts of curcumin present before and after treatment of tissue extracts with conjugate-hydrolyzing enzymes29. Thus, gut metabolism contributes substantially to the overall metabolic yield generated from curcumin in vivo. In human intestinal fractions, conjugation with activated sulfuric or glucuronic acids was much more abundant, whereas conjugation in human hepatic tissues was less extensive, than in the rat tissues30.
Although p.o. administered curcumin has poor bioavailability and only low or non-measurable blood levels were observed31, this route of administration inhibits chemically induced skin and liver carcinogenesis32,33. Oral administration of curcumin also inhibits the initiation of radiation-induced mammary and pituitary tumors34. Similarly, in a study to assess the curcumin levels in the colorectum, a daily dose of 3.6 g curcumin achieves pharmacologically effective levels in the colorectum with negligible distribution of curcumin outside the gut35.
Earlier Shobha et al36 had observed that administering piperine along with curcumin enhances the bioavailability of curcumin. However, the level of enhancement was only modest and no curcumin could be detected after 3 hours even when supplemented with piperine.
Although some questions remain unanswered regarding the pharmacokinetics of curcumin in humans, there is no denying the fact that considerable proportion of ingested curcumin is excreted through feces and at least about one-half of absorbed curcumin is metabolized. The quantity of curcumin that reaches tissues outside the gut is probably pharmacologically insignificant. Several studies have failed to demonstrate the positive in vitro results with curcumin in in vivo animal and human studies due to lack of absorption of curcumin after oral administration. To provide the clinical benefits, curcumin must be absorbed from its oral route of administration at a suitable rate, be distributed in adequate concentration in the blood and remain in the system for a sufficient period at an effective concentration level.